1. Field of the Invention
The present invention relates to a light-emitting-diode (LED) and method of manufacturing the same, and in particular to a three dimensional (3-D) LED stack and method of manufacturing the same.
2. The Prior Arts
The light-emitting-diode (LED) is a special type of semiconductor diode developed in 1960'. The simplest configuration of LED includes a pn junction formed between a p-type semiconductor and an n-type semiconductor. When a current flows through the pn junction, charged carriers such as electrons and holes are generated. Then, the electrons and holes are combined to produce and emit photons to achieve release of energy. Recently, a high performance LED further includes one or more light emitting layers, sandwiched between the p-type and the n-type semiconductor regions, to improve the light emitting performance. The one or more light emitting layers mentioned above are used to obtain the light emitting wavelength desired. The basic structure of LED includes a small block of stacked layers of materials mentioned above, and is usually referred to as a die. The die can be placed on a frame or a substrate for electric contact or mechanical support, and it is glued for protection.
To the light-emitting-diode (LED), the wavelength of the light emitted depends on the difference of energy band gaps of the light emitting layers. The semiconductor compound suitable for used as material of light emitting layer has energy gap that can produce and emit infrared light, visible light, or ultraviolet light. AlGaInP is a typical material used for light-emitting-diode, due to its high quantum light emitting efficiency, namely high illumination, and adjustable colors. The energy gap variations of alloy (AlxGa1-x)1-y InyP depend on the x and y values of the compound, and color range of AlGaInP LED is between green light and red light. In general, AlGaInP LED is produced on a lattice matching GaAs substrate, and is formed by epitaxy, such as Metal Organic Chemical Vapor Deposition (MOCVD).
In 1990s, the GaN (Gallium Nitride) series violet light, blue light, and green light LEDs are developed and produced. For the GaN-series direct energy gap semiconductor, the energy gap difference is about 3.4 eV. The wavelength of the photons produced by recombination of electrons-holes in Gallium Nitride is 360 nm, namely in a range of ultraviolet light. For visible light (green light, blue light, and violet light) LED, InzGa1-zN can be used to produce the light emitting layer, sandwiched between a p-type GaN layer and an n-type GaN layer. The wavelength λ, of light emitted by InzGa1-zN series LED can be varied depending on z value of the compound. For example, to pure blue light (wavelength λ=470 nm), z value is 0.2. Similarly, Gallium Nitride LED must be produced on a lattice matching sapphire or SiC substrate, and is formed by epitaxy, such as Metal Organic Chemical Vapor Deposition (MOCVD).
In the past, quite a lot of researches have been conducted to develop white light LED to replace the conventional light source. Presently, the following approaches are used to produce white light LED:
(1) Put separate and independent red light, green light, and blue light LEDs into a “light source”. Utilize various optical elements to mix the lights emitted by the red light, green light, and blue light LEDs. However, since different color LEDs require different operation voltages, hereby requiring multiple control circuits. Moreover, the service lives of different color LEDs are different. So after long period of usage, some of the LEDs will deteriorate or just fail, thus color of the mixed light will change evidently.
(2) Utilize phosphor material to convert part of lights of the short wavelength into lights of long wavelength. In this respect, the most frequently used approach is to put yellow phosphor powder around the blue light InGaN LED chip. The yellow phosphor powder is made by doping material Ce into yttrium aluminum garnet crystal, namely YAG:Ce. Part of the blue light emitted by the InGaN LED chip is converted by YAG:Ce into yellow light. However, the white light produced in this approach only includes lights of two colors: blue light and yellow light; thus that is only applicable to the indication lamps.
(3) In order to produce white light, utilize the ultraviolet (UV) light produced by ultra-short wavelength LED to agitate several different phosphor materials, to produce mixed lights of various colors. The shortcoming of this approach is that, the service life of UV LED is rather short compared with other LEDs. Furthermore, the UV light emitted by LED is hazardous to the human body. And at present, most of packaging materials are not able to effectively shield off the UV radiation.
In the prior art, white light LED light source of high efficiency and good chroma have been developed. For example, a photon recycling concept is disclosed in the following article to produce high brightness white light LED: Guo et al., “Photon—Recycling for High Brightness LEDs”, compound semiconductor 6(4) May/June, 2000. Photon Recycling refers to a process of short wavelength photons being absorbed by a light emitting material, so that this material is able to emit long wavelength photons. Basically, the photon recycling semiconductor (PRS) LED is able to emit white light effectively to a brightness of 330 lumen/watt. However, the drawback of PRS-LED is its rather low color rendering index.
The double color PRS-LED proposed by Guo et al. includes a first color light source and a second color light source. The second color light source is provided with a second light emitting layer. The first color light source is used to produce blue light. The blue light thus produced is directed toward the second light emitting layer, so that part of the blue light is absorbed, and then yellow light is produced in a light re-emitting process. Basically, the double color photons generated by PRS-LED are similar to that generated by LED applied with phosphor material. However, its difference with the LED applied with phosphor powder is that, the second color light source includes phosphor semiconductor material (AlGaInP), that is bonded directly onto the first color light source wafer. Therefore, the double color PRS-LED can be produced directly on a wafer.
Refer to FIG. 1 for a cross section view of a PRS-LED structure according to the prior art. As shown in FIG. 1, the PRS-LED structure 10 includes a transparent substrate 12; a first color light source; and a second color light source. Wherein, the transparent substrate is for example a sapphire substrate, the first color light source and the second color light source are on opposite sides of the substrate 12. The first color source includes a p-type GaN layer 14, an active layer 16 formed by InGaN, and an n-type GaN layer 18. The layers mentioned above are formed on the substrate 12 by means of epitaxy growth. The second color light source includes AlGnInP layer 22. The AlGnInP layer 22 is formed on a GaAs substrate (not shown) by epitaxy growth. Then, gluing material 24 is used to glue it onto the substrate 12. Subsequently, the GaAs substrate is removed through chemical-assisted polishing and selective wet etching. Then, the first color light source is patterned, to form an n-type contact 26 and a p-type contact 28 by using aluminum. The n-type contact 26 is deposited on region of n-type GaN layer 18, while p-type contact 28 is deposited on region of p-type GaN layer 14.
The output of the first color light source is produced when a current is flowed into the active layer 16, such that the wavelength of light emitted by the first color light source is 470 nm (blue light). In operation, part of light emitted by the first color light source is absorbed by the AlGnInP layer 22, then light is re-emitted (or re-utilized) to have longer wavelength. The composition of the AlGnInP layer 22 is so selected that it can re-emit light of wavelength 570 nm (yellow light). Since the colors of lights emitted by the first color light source and the second color light source are mutually complementary, therefore, to the human eye, the light output after combination appears to be white light. However, for the PRS-LED structure mentioned above, the emitted white light includes blue light of 470 nm and yellow light of 570 nm, wherein, red light of 650 nm is not included, thus leading to poor color rendering index.
Therefore, presently, the design and performance of PRS-LED structure is not quite satisfactory, and it has much room for improvements.